This invention pertains to a method for obtaining an effective capping layer when applied in the fabrication of integrated circuits. More particularly, the invention focuses on the creation of a cobalt diffusion barrier by electroless deposition, followed by nitridation of the surface of this diffusion barrier, to create a more effective diffusion barrier.
The use of copper as the main on-chip conductor in integrated circuits is now mainstream. Compared to aluminum, copper has lower electrical resistivity and better electromigration and stress-void resistance. However, the substitution of copper in place of aluminum has created various technical challenges.
Among these challenges is copper's high diffusivity in silicon dioxide and other dielectric insulating material used in integrated circuits. Copper diffusion into the surrounding insulator can cause line-to-line shorts. Furthermore, if copper manages to diffuse through the silicon oxide layer and enter into the silicon lattice, it will create impurities that will degrade transistor performance and cause junction leakage.
Another challenge to using copper is that, unlike aluminum, copper does not adhere well to dielectric materials. This adhesion problem can cause voids in the copper lines in areas of the copper-insulator interface. As electrical current is passed through the copper, copper will tend to migrate away from these areas, creating even larger voids and degradation of copper lines. This is the phenomenon of electromigration.
FIG. 1A further illustrates the two issues mentioned above with an illustrated cross section of a portion of a simple dual damascene device. A copper plug 105 connects two copper lines 101 and 103 in separate layers. There is dielectric material 109 that separates the copper lines in regions that the copper plugs do not exist. Conventionally, diffusion barriers (119, 121 and 123) separate the copper lines and plugs from surrounding dielectric material in the same metalization layer (125, 109 and 127, respectively). These diffusion barriers are typically titanium nitride or tantalum nitride. Over time, copper ions from the copper line 103 will diffuse through the dielectric material 109 in this device. In addition, the electromigration of copper and void creation will tend to occur in the copper-dielectric interface regions, especially in corner regions 113.
FIG. 1B depicts a similar device as FIG. 1A, but with a capping layer 115 comprising a diffusion barrier material. Generically, a diffusion barrier is a thin layer of material that is deposited between the conductive and insulating layers in a wafer to prevent metal diffusion into non-conductive areas. Diffusion barriers typically take the form of diffusion barrier layers that line the trenches and vias provided in a dielectric layer during damascene processing. They are also commonly used in “capping” layers that cover the “top” surfaces of deposited copper lines. Ideally, the capping layer can help to alleviate the problem of electromigration. As shown in FIG. 1B, capping layer processes require a capping layer 115 be deposited selectively over the metal surfaces. A capping layer is formed on top of the copper line 103 before the deposition of the dielectric material 109 and copper plug 105. So, compared to the device shown in FIG. 1A, the copper in the device of FIG. 1B is prevented from diffusing into surrounding dielectric and the copper-dielectric interface regions are less susceptible to void formation due to copper electromigration.
Diffusion barrier materials should exhibit low resistivity, nominally less than 1000 μΩ-cm. This is so that the resistivity of the composite copper/barrier interconnect area is low and the total line resistivity remains low. It should adhere well to both dielectric material and copper. Typically, copper diffusion barriers contain refractory metal compounds since these metals exhibit the low resistivity, adhere well to copper and can maintain good barrier properties at higher temperatures. Titanium, tantalum and tungsten metals and their nitrides are commonly used. More recently, other refractory metal compounds consisting of nickel, molybdenum, and cobalt have been investigated.
One must also take into account the processes and methods of forming the diffusion barrier. For example, the barrier material must be an effective copper diffusion barrier at all post-copper deposition temperatures. It must also be able to deposited and/or annealed at temperatures below which copper lines will start to migrate, typically no greater than 400 degrees Celsius. In addition, if the method for depositing the barrier layer results in a film that is too thick, it will take up too much of the space in the line trench. This would decrease the amount of copper that could be deposited and would impact the resistance of the interconnect line. Furthermore, if the method used results in a layer with poor step coverage. unacceptable void areas will be formed, especially in high aspect ratio trench regions prevalent in modem copper devices.
As copper line technologies continue to rapidly evolve and keep pace with the continual movement to smaller, higher performing devices, there is a continuing need for improving diffusion barriers and finding better methods for depositing them. For this reason, those in the industry continue to focus much of their work with in this area.